This section is intended to provide a background or context to the invention recited in the claims. The description herein may include concepts that could be pursued, but are not necessarily ones that have been previously conceived or pursued. Therefore, unless otherwise indicated herein, what is described in this section is not prior art to the description and claims in this application and is not admitted to be prior art by inclusion in this section.
To address the vexing problems of depleting fossil fuel reserves and rising greenhouse gas levels, renewable energy sources such as solar radiation, wind, and waves are undergoing intensive investigation. All of these renewable sources are intermittent, and will require energy storage technologies, such as batteries, to produce a continuous power flow. Electric vehicles (EVs) can also play a role in solving these challenges, but again, substantial improvements in battery performance are required to realize their full potential. Lithium-ion batteries (LIBs) are promising candidates for fulfilling the energy storage requirements of renewable power and EVs. LIBs, consisting of a LiCoO2 cathode and a graphite anode, have dominated the consumer electronics market since their introduction in 1991, primarily due to their superior energy and power density compared to other secondary batteries. However, to meet the demanding requirements for EVs, the specific energy of LIBs must increase by 2-5 times from the current value of 150 Wh/kg. As a consequence, the quest for higher-performance electrode materials with increased power, energy density, lifetime, and safety is ongoing. In particular, nanomaterials are regarded as having great potential to enhance LIB performance as a result of their reduced dimensions. Of the many classes of nanophase materials investigated to date, metal sulfides are relatively unexplored, but show great potential to serve as either electrodes (e.g. MnS, FeS/FeS2, CuS, and ZnS), or lithium-ion conducting solid electrolytes (e.g., Li10GeP2S12) and consequently they are a promising avenue of research.
Lithium sulfur batteries are being considered as an alternative to conventional lithium ion due to the higher theoretical capacity (2567 Wh/kg for Li—S vs. 387 Wh/kg for Li-ion). In addition, sulfur is inexpensive, abundant, and environmentally-friendly. Despite their numerous advantages, daunting technical challenges must be overcome before Li—S batteries can be commercialized, including: (1) poor electrode rechargeability and limited rate capability due to the insulating nature of S and Li2S; (2) rapid capacity fading due to the formation of soluble polysulfides Li2Sn (3≦n≦6); and (3) a poorly controlled Li/electrolyte interface (i.e., sulfur dissolution into the liquid electrolyte of Li—S batteries).
The lithium metal anode of Li—S batteries has also been identified as a potential risk in practical use due to the well-known Li dendrite growth during cycling which can lead to catastrophic failure. Furthermore, limited reserves of Li resources impose another concern in view of the colossal demands of automotive transportation. A viable solution is substituting S with Li2S and the latter as the Li source is capable of coupling with many more promising and economic anode materials such as Si and Sn. As a consequence, Li2S is currently undergoing intensive investigation.
The reductive Li2S has a capacity of 1166 mA·h/g, four to five times greater than intercalation-based cathodes of LIBs. Li2S is intrinsically insulating, both electronically and ionically. To compensate for this deficit, conducting materials, such as carbon and metals, have been mixed with the Li2S via ball milling or other techniques. Studies have also explored the use of solid state electrolytes to improve safety, ionic conductivity, or both. These studies unanimously demonstrated that dimensional reduction of the Li2S is crucial to achieve high capacity retention, cycling stability, and rate. They established that nanosized Li2S (nano-Li2S) composites are particularly preferable to their microsized counterparts. One study disclosed that nanosized Li2S has higher ionic conductivity compared to the bulk form, and they claimed that this high ionic conductivity was responsible for the improved cycleability that they observed. Furthermore, nano-Li2S is believed to alleviate pulverization of the cathode from repeated cycling, and to offer shorter transport pathways for electrons and ions. To date, Li2S has been mainly synthesized via solid-state reactions and solution-based methods. These approaches lack the precision required for careful dimensional control in nanophase composites. Consequently, precise and nanoscale dimensional control over the Li2S component in composite cathodes is paramount for these Li—S systems.
Metal sulfides represent an important class of functional materials that exhibit exceptional electrical, optical, magnetic, and mechanical properties. Furthermore, the chemical properties of metal sulfides have stimulated their use in heterogeneous catalysis. Metal sulfides have also demonstrated excellent electrochemical properties, and this quality offers great potential for their use in energy conversion and energy storage devices. Consequently, metal sulfides have attracted great attention, and numerous techniques have been devised for synthesizing metal sulfide materials, including solution-based methods, chemical vapor deposition (CVD), and physical vapor deposition (PVD). Gallium sulfide (GaSx) has two stable forms: GaS and Ga2S3. Both forms are wide-band-gap semiconductors, making them promising candidates for optoelectronics and photovoltaics. Moreover, GaSx is ideal for passivating GaAs surfaces in high-mobility semiconductor devices. Previous reports have described the deposition of GaS and Ga2S3 thin films using PVD and CVD. In addition, nanostructured forms of GaS and Ga2S3 have been reported including nanoparticles, nanotubes, flowerlike structures, and nanowires and nanobelts. Unfortunately, PVD and CVD do not typically provide the necessary control over thickness and composition required for the precise synthesis of nanostructured materials.
Recently, atomic layer deposition (ALD) has emerged as a versatile technology for fabricating thin films and nanostructured materials. ALD utilizes alternating exposures between two or more precursor vapors and a solid surface to deposit material in an atomic layer-by-layer fashion. The unique aspect of ALD compared to conventional chemical and physical vapor deposition (CVD and PVD), is that the different precursors are supplied individually, and they react with the surface in a self-limiting manner. The self-limiting nature of ALD provides atomic-level precision over the film thickness and composition, and it yields exceptionally uniform films over large areas and in complex geometries. In particular, ALD films are exquisitely conformal and uniform, even on high surface area or high aspect ratio substrates, and the film thickness and composition can be controlled at the atomic level. Because of these benefits, ALD is being applied in energy devices, catalysis, medical and biological devices, plasmonic devices, nano- and microelectromechanical systems, and novel nanostructured materials. Because of their unique properties and numerous potential applications, metal sulfides are gaining interest in the ALD community.
A need exists for improved technology, including technology that may address the above problems, namely by providing an ALD mixed-metal solid state electrolyte or and method for manufacturing such a solid state electrolyte that permits the infiltration of porous electrodes and the deposition of thin, conformal films. The materials may also be used for protective electrode coatings.